Regardless of the pavement composition, repairs are inevitable, especially in wet climate areas and those susceptible to repeated freeze-thaw cycles. For many years cold patch compositions have been an integral part of a short-term solution to the problem of distressed roadway pavement. Such materials have been used by various municipal, state, federal, and independent highway agencies to temporarily repair potholes, cracks, ruts, etc. (For the purpose of this discussion, the term "cold patch" will be used to refer to asphaltic repair material of this sort, made without heating, otherwise known by a variety of terms, including pot- or chuckhole patch mix, bituminous concrete winter mix, stockpile patching material, and cold mix.)
Typically, cold patches comprise graded aggregates uniformly-coated with a pre-formulated asphaltic bitumen solution or emulsion, prepared by dissolving an asphalt in an appropriate solvent, usually a petroleum distillate such as mineral spirits, naphtha, kerosene, fuel oil, flux oil or the like. (Asphalt-water emulsions tend to be used less-frequently than their solvent counterparts.) Surfactants are often added to promote a variety of desirable properties such as internal cohesion and asphalt adhesion to the aggregate surface.
After loose dirt or debris is removed from the distressed area, cold patch is applied liberally such that it is level with the roadway surface after compaction. In many instances "throw and go" application techniques are utilized, whereby the cold patch is simply shoveled from the back of a truck onto the distressed area, leaving it to subsequent motor vehicle traffic to provide the necessary compaction. As imagined, it is desireable that a cold patch remain "workable" in any weather condition, regardless of the length or type of outdoor storage. ("Workable" is defined as being easily applied via shovel.)
Casual observation shows pavement repair to be an on-going exercise in most communities. Because complete pavement restoration is extremely-costly, it is advantageous to utilize cold patch repair whenever possible. As a result, a tremendous amount of cold patch material is consumed each year by various independent and government highway agencies. While cold patch compositions are relatively inexpensive, cost is still an overriding concern given the volumes used.
A concurrent problem relates to the accumulation of asphalt roofing wastes. Each year, according to a recent estimate, approximately 70 million squares of roofing shingles are produced in the United States. A square, by definition, is material sufficient to cover 100 square feet of surface area without overlap, typically contains 80 shingles, and depending on the particular shingle composition, weighs between 210-225 pounds. Shingles produced after 1980 are 20-30% asphalt by weight, with the remainder fiberglass mat, roofing granules, filler, and back-surfacing materials. Those produced prior to 1980 contain roughly 1.5 times the amount of asphalt.
Significant waste accompanies the manufacture of asphalt shingles. Broken and defective shingles are discarded. Production of a standard three-tab shingle produces cut-outs and trimmings which total nearly 1% of its total weight. Based on annual shingle production figures (and using post-1980 composition data), approximately 77,000 tons of shingle cut-outs/trimmings and 20,000 tons of asphalt are generated each year. The total amount of broken and defective shingles discarded annually is of a comparable magnitude.
An additional facet of the situation is that a significant amount of previously-applied shingles are recovered each year. The typical residential roof may be re-shingled up to three times before all old shingles are removed. On an annual basis approximately 4.2 billion pounds of asphalt are generated each year in this manner--in addition to the manufacturing wastes mentioned above.
The annual amount of annual waste attributable to shingles provides only a glimpse as to the real scope of the problem. Other types of asphalt roofing materials include asphalt-saturated organic felts, asphalt-impregnated glass and polyester mats, rolled roofing products such as ply sheets, modified bitumen membranes and the like, as well as commercial build-up roofs (alternating layers of asphalt and prepared rolled roofing products on commercial buildings). Each has associated with it a significant amount of waste. (For the purpose of this discussion, the term "asphalt roofing wastes" will be used to refer to wastes generated through the manufacture and/or disposal of these and other like materials.)
For many years, asphalt roofing wastes have been land-filled. What once seemed to be a safe and sound "solution" spawned a variety of more worrisome concerns, foremost among which is the creation of permanent landfills--land which over time becomes non-reclaimable. Furthermore, with an increasingly-larger population and ever-growing volume of solid waste, the number of available landfills has dwindled to the point where most states now ban certain types of refuse. It is no longer environmentally-wise or economically-feasible to continue landfill disposal of asphalt roofing wastes.
The search for an efficient and high-volume use for asphalt roofing wastes meeting the requirements stated above has been an ongoing concern in the art. One approach, with application to roadway surfaces and described in U.S. Pat. No. 4,325,641, relates to a method of recycling asphalt shingles whereby a hot mix pavement end product is derived which comprises about 0% by weight recycled asphalt shingle waste and about 0% by weight asphaltic oil. A similar approach, discussed in U.S. Pat. No. 4,706,893, also relates to a method of recycling asphalt waste such that a hot mix pavement end product comprises about 5-6% asphalt, of which only about 40% of the asphalt component --or 2% of the total weight of the composition--is derived from recycled shingles; the remainder made of a paving grade liquid asphalt.
Both patents mentioned above provide a method and apparatus whereby asphalt roofing waste may be used as a minor additive in an asphalt paving composition. The amount of asphalt roofing wastes also used and recycled in this manner is a very small percentage of that available. The reuse limitation is inherent to the nature of asphalt pavement materials.
As mentioned above, pavement comprises no more than 6% by weight asphalt. If more is used, the pavement surface becomes oily and slick at normal summer temperatures, due to excess asphalt actually oozing or "flushing" out of the composition. An asphalt pavement is defined largely by its aggregate component. The aggregate is of a size, cut, and porosity which imparts to it a relatively large surface area per unit volume. In order to ensure 100% coating, a high-penetrating asphalt is needed, one which is readily fluid at process temperatures of 300.degree.-400.degree. F. Oxidized asphalts, found in roofing products, are not of this type and present process problems and subsequent mix design difficulties if substituted for conventional paving grade asphalt. As a result, no more than 40% of the asphalt component--or 2% of the total weight of the pavement compostion--may be derived from recycled asphalt roofing wastes.
Even if the use of more recycled asphalt roofing waste was found to provide a suitable hot mix pavement composition extension of the technique to a pavement repair compostion would be no more feasible than it is now. While a "hot patch" would be the material of choice, it is not practical in cold temperatures or rain conditions. Constant heating and related process requirements are too costly for minor repair work. Substituting use of a solvent for heat, cold patch compostions are economically-feasible and workable in a variety of weather conditions.
The search for a cost-efficient and effective cold patch composition, along with a beneficial use of recycled roofing waste, has been an on-going concern in the art. One attempt has been to blend a very small amount of asphaltic waste, including asphalt roofing wastes, with a pre-made commercial cold patch material, as an economizing measure. A heat source melts the asphalt waste into the pre-made cold patch. While this approach represents a use, albeit limited, for recycled asphalt roofing wastes, it does not provide, by any definition, a cold patch material. A cost-prohibitive heat source is required through application, irrespective of weather conditions, and the material obtained is completely unsatisfactory in terms of workability and performance.
The prior art has associated with it a number of obvious and significant problems and deficiencies, regardless of whether a minor portion of the bitumen component is derived from recycled asphalt roofing wastes. Most cold patch problems are related to poor performance and result from the type of asphalt and methods of preparation currently employed.
A major problem of the prior art stems from the fact that a cold patch composition must be designed in such a manner that pertinent physical properties can be measured with some degree of precision and correlated to actual roadway performance. Many governmental units concerned with this situation, including state highway agencies, have adopted such an engineering-based approach. Several are used, but the most widely-accepted approach is known as the "Marshall Method".
The recent trend is to transfer responsibility for cold patch design from specifying agencies to the independent contracting industry. Because of the current emphasis on engineering performance properties, a cold patch can no longer be sold merely on the basis of a "recipe" formulation which does not correlate well to quality and performance. The contractor engaged in competitive bidding must, therefore, be able to provide performance properties when samples are submitted.
The problem arises in that no conventional cold patch composition has engineering performance properties in the "green state"--prior to solvent evaporation, as would exist at the time of pothole application. As such, most cold patches of the prior art do not physically withstand the test procedures employed such that measurements can be made. Those which do provide measurements give results which correlate directly to the poor roadway performance eventually observed.
Another related problem is that cold patches of the prior art perform poorly in terms of cohesion and adhesion. Cohesion refers to the internal integrity of the cold patch. Inadequate cohesion is typically evidenced by the scattering of cold patch as one drives over a newly-patched pothole. Over time, the cold patch becomes strewn in all directions instead of remaining in the pothole. Adhesion is the bonding interaction between the cold patch and the pavement structure. Inadequate adhesion causes the cold patch to eventually work away from the pavement and recreate the pothole.
A related concern is the prolonged curing time required by conventional cold patches. Cohesive integrity is achieved only upon evaporation of the solvent used in the cold patch preparation. Evaporation time is often extended, often up to days, by geographic and climatic factors. Because it is extremely undesireable to re-route or stop traffic for every minor repair, much of the cold patch applied is often scattered by vehicle traffic before it is sufficiently cured.
The result of long curing times and inadequate cohesion or adhesion, is the likelihood of multiple repair of the same distress areas over the course of a typical repair season. Inasmuch as cost is a tremendous concern to any governmental body or department working against budgetary constraints, multiple repairs represent an unneeded expense.
Another significant drawback to cold patches of the prior art relates to the asphalt used therein. Soft asphalts, which are very temperature-susceptible, are typically used. As thermoplastic materials, they become hard and cement-like upon a drop in ambient air temperature, but excessively-fluid upon a temperature rise. Inability to remain stable over a range of temperatures adversely effects workability, especially at colder temperatures. Application becomes time-consuming if the cold patch is hard and difficult to shovel.
Performance is also effected adversely by temperature-susceptibility. As with pavement materials, a common problem of conventional cold patches, one readily-observed on a hot summer day, is flushing. Repeated traffic also "ruts" or otherwise deforms softened cold patch. Ruts intensify with repeated traffic and each subsequent temperature rise, invariably necessitating further repair.
Various filler and fiber materials may be included to enhance performance by stabilizing and strengthening the cold patch. However, addition of such materials is costly. Because of the large volume of cold patch used, any increase in cost is prohibitive. Unfortunately, many agencies opt for a cheaper, though less-capable, cold patch composition.
If filler and fiber materials are added, inefficiencies abound. Because they tend to have a dust-like consistency, a significant amount is often lost under the negative air pressure conditions typically employed during mixing, further raising the effective cost. Dispersal of the filler and fiber throughout the cold patch is problematical and achieved only after much effort. Non-uniform dispersal provides a non-homogeneous cold patch with inconsistent performance, which raises the effective cost even more by necessitating multiple repairs.
In summary, there are a considerable number of drawbacks and problems relating to cold patch materials for roadway repair. There is a need for a improved cold patch, especially one which takes advantage of the environmental and commercial benefits associated with recycled asphalt roofing wastes.